In the wet end of a papermaking machine, the sheet of paper is formed on one or between two papermaking machine fabrics, depending on the machine type. Here a suspension of approximately 1% fiber substances and auxiliary substances and 99% water is uniformly delivered onto the papermaking machine fabric. The solid components contained are separated from the water by a filtration process. By using different drainage elements as far as the end of the wet end, usually a solid portion or dry content from 20 to 22% is attained. The paper produced in this way is already sufficiently solid so that it is transferred to the press end and can be further dried. In modern papermaking machines such as gap formers, the technique for executing this filtration process has been improved to such an extent that at speeds of 2,000 m/minutes and more, much less than one second is necessary for this purpose.
While a series of quantities in paper technology, such as the degree of whiteness, tensile strength and extension, are determined by the raw materials and auxiliary substances used, the sheet forming fabrics have a major effect on the substance weight profile and transverse thickness profile, the fiber retention and the fabric marking which is visible in the paper. In particular, in the area of graphic papers, these quality requirements are high. These quality requirements are opposed by the fact that visibly lower quality raw materials such as scrap paper are used and that by reducing the substance weight the attempt is made to save raw materials and thus money.
In the development of sheet forming fabrics for papermaking machines, a reaction rather early to these trends and various solutions was implemented to reach a compromise between a fineness of the paper side as high as possible and a machine side as durable and coarse as possible to achieve a high wear life time and overall high stability of the wire cloth.
To meet these requirements, double-layer wire cloths were developed characterized by a longitudinal thread system with at least two transverse thread systems, at least one transverse thread system being woven in solely on the paper side and the second exclusively on the machine side. Some embodiments of these sheet forming fabrics can be found among others in the following patent documents: EP 0 186 406 A3, DE 31 43 433 A, DE 25 40 490 B2, DE 22 63 476 B2, DE 38 17 144 A1, DE 38 01 051 A1, DE 39 10 019 A1, and in DE 41 07 633 C2.
Although this type of sheet forming fabric is also often used today, all these cloths are characterized by a comparatively low water permeability due to the machine direction threads which are located tightly next to one another. Furthermore, with this type of cloth it is not possible to produce an ideal paper side recognized as a linen weave and to join it to a lower cloth. Accordingly, frequently unwanted markings occur in the paper.
To remedy these disadvantages, efforts have been made to develop a fabric in which the paper side has a linen weave. The three-layer composite cloths have at least two separate cloth layers, with the upper layer characterized by the linen weave and connected in a suitable manner to the other cloth layers. To join the layers to one another, in the known solutions separate binding threads are used forming both the longitudinal threads and also the transverse direction threads. Furthermore, this connection can also be produced in that longitudinal and transverse thread systems of one cloth are tied into the other cloth by the binding threads changing over from one cloth layer into the next and vice versa. Some examples of this can be found in the following documents: DE 33 18 985 A1, DE 42 29 828 C2, DE 29 17 694 C2, EP 01 093 096 B1, EP 0 069 101 B1, EP 0 097 966 B1 and WO 93/00472.
With these types of known cloths, the uniformity of the upper side could be improved such that the marking left in the paper is much less than for a two-layer fabric. Also, permeability was maintained for a comparatively large number of threads on the paper side. This led to increased use of three-layer fabrics for the indicated graphic papers.
But the multilayer structure greatly increases the thickness of the papermaking machine fabric, by which more material can be intercalated in the cloth and the drainage performance is reduced in this way. Another disadvantage of the three-layer composite cloth relates to the joining of the layers. The individual cloth layers connected to one another by binding threads extending in the papermaking machine are often stressed in bending by deflections, relative to a neutral phase in the middle of the fabric, one layer undergoing stretching, at the same time the fabric layer opposite the neutral phase being shortened. Loosening of the cloth connection generally then occurs. This loosening can lead to shifting of the layers in the advanced stage with separation of the cloth layers occurring in the extreme case, leading to the papermaking machine fabric becoming unusable.
In DE 100 30 650 C1, to counteract layer separation, the machine direction threads can be used as binding threads with the result that by changing the longitudinal thread or machine direction threads from the paper to the machine side and vice versa the stretching and shortening of the two cloth sides are equalized. Even this type of weave cannot compensate for the structure-induced higher thickness of the composite cloth with its reduced drainage performance.